Boiling Fuel

[Digger 258]

Mowed for about an hour in 85 degrees and when I parked to move the trampoline, I heard a hissing from the fuel tank cap (it's a steel tank in the lid, with a steel cap). When I peered into the tank the gas was at a full boil. I was hesitant to continue mowing after that, so I waited an hour and searched the forum in an attempt to smooth out my fears - hoping to find that "everyone's gas boils, this is normal!"

I read about wrapping or re-routing fuel lines (not sure I can) and new fuel filter (tomorrow's plan), but other than that, should I be concerned? I rebuilt the carb a few months back thinking I was running lean when my exhaust and expanded metal in front of the exhaust were glowing red hot at night, but I'm not sure it's running any cooler now.

Not sure if it's even safe to use, since boiling gasoline just seems wrong . . . I appreciate your thoughts/experience.

Regards

 

[dragoneggs]

If it was truly boiling it hit 180degF! May want to check that the tank is insulated from the heat source.

 

[Marks481]

I had problems with that.

My ultimate solution was to remove the fuel pump from its mount one the left cylinder head (close enough that heat radiating from the exhaust cooks it) and mount it on the left side of the engine compartment. Also put silver thermal sleeving over the fuel hose where it passes near the exhaust manifold.

The mechanical fuel pump is driven from impulse pressure through a 1/4" tube connected to the crankcase, so you can relocate the fuel pump wherever you want it.

After having my second fuel pump go bad, I recently replaced the mechanical fuel pump with an electrical fuel pump and am happy with the results.

 

[TomSeller]

Holy Crap! Was it sitting in the sun? What make is the mower? How hot was the fuel? Some components of gasoline vaporize off at fairly low temperatures.

 

[Digger 258]

I was indeed mowing in the sun, and the gas was at a full boil when I stopped. Thanks for the tips Tmarks, I'll find some thermal sleeving and re-route the fuel pump and see what that gets me. I thought about putting another fan in the side of the lid, but then again, maybe I'd be just covering up what is the real cause, the engine running too hot.

 

[MuncyBob]

I'm assuming you are running the 425 with the stock PT exhaust? If so, I would suggest you search the threads for the installation/mod of installing the Robin muffler along with an additional side exhaust fan. I still get a little bubbling at he bottom of my fuel tank but it's certainly not boiling.

The exhaust mod drastically reduced my tub temps. I also insulated my fuel line and one of these days I need to relocate the pump too.

 

[Oldoak]

Not sure if it's even safe to use, since boiling gasoline just seems wrong . . .

I would say not! Definitely not under that condition. Boiling gasoline? You were virtually sitting on a BOMB. With the vapors being released, you were very, very fortunate they didn't ignite on that glowing red hot metal or when you opened the gas cap to peek in.

I wouldn't continue to press your luck.

 

[agjones]

My subaru engine didn't last long enough for me to see that on the PT 425, but I've experienced it on another machine. And I got proactive w the PT once I realized how spectacularly hot the pipe was.

That exhaust definitely needs shielding...the tank could use it too. I applied a thin sheet of aluminum to each side of the steel heat shield vertical and underneath separated by a few washers as spacers.

Make sure the gas cap is venting properly (air comp clean) excess pressure in the tank can add to the problem, and is dangerous when you can't resist peeking...

I found that the oil cooler fan did a pretty good job clearing heat from the tub...but aluminum and an air gap did much better.

 

[Marks481]

...excess pressure in the tank can add to the problem, and is dangerous when you can't resist peeking...

+++1

DAMHIKT!

 

[ruffdog]

Here is a good article explaining many aspects of fuel. I have noticed that E10 will boil sooner than E0 too. If you run winter grade fuel, it will boil much sooner than summer grade.
Notes from the field: Ethanol, Vapor Lock and other Gas Woes - Military TraderMilitary Trader